1. Field of the Invention
The present invention relates to a laser remelting process to modify surface properties of a zirconia coating on a metal article.
2. The State of the Art
For the purpose of raising working temperature and operation efficiency at high temperatures, an engine part needs a surface coating acting as a thermal barrier to prolong its life time and performance. Plasma spraying of abrasive and refractory ceramic materials on a metal substrate is one of the techniques to apply a coating layer on engine parts.
Sim et al. ("Superalloy II", John Wiley & Sons, Inc., 1987) have divided thermal barrier coatings (abbreviated as "TBCs") into two categories: one is diffusional coating, e.g. pack cementation or chemical vapor deposition (abbreviated as "CVD"); the other is overlay coating, e.g. plasma spray. It would be beneficial to operate the engine part at higher temperature. The ceramic top coating on a metal part is necessary for preventing the metal substrate from overheating. The effects of lowering substrate temperature depend upon the thickness of the coating and the thermal conductivity of its top and the coatings. The greater the temperature difference (.delta.T) between the environment and the engine part is achieved, the better the protection and efficiency that are provided. As a consequence, the amount of inlet cooling gas can be reduced and operation efficiency is improved greatly.
The preparation of TBCs known to the artisan is conducted with a high temperature (5000.degree. C. to 30000.degree. C.) air plasma touch. Ceramic powder is melted in the plasma and projected onto a metal article. The partially-melted or fully melted ceramic particles are quenched and adhere to the surface of the article. However, the mechanical bonding of the coating layer deteriorates due to the thermal expansion mismatch between the ceramic layer and the metal matrix. Large interfacial stresses are generated. The stresses can be reduced by applying a bond coat (e.g. MCrAlY metal alloy layer, reported by G. W. Goward, Materials Sci. & Tech., 2[3] (1986) 194-200) between the ceramic top coat and the metal substrate. However, the structure of the TBCs is still porous and allows oxygen to pass through to the metal bond coat. The porous TBC spalls when the multilayer structure is exposed to oxidizing and corrosive environment. If the bond coat can be shielded from the corrosive media, including oxygen, the lifetime of TBCs can be effectively extended. Therefore, many modifications have been proposed and implemented, and have proven to be effective to some extent, for example: pre-oxidation of the bond coat; pre-aluminization of the bond coat (Wei-Cheng Lih, Ph.D. Thesis, National Cheng-Kung University, Tainan, Taiwan, R.O.C., 1992); application of denser ceramic top coat by using low pressure plasma spray (LPPS); and laser sealing of ceramic top coat (K. Mohammed Jasim, R. D. Rawlings, and D. R. F. West, J. Mat. Sci., 27 (1992) pp. 3903-3910 or A. Smurov and Y. U. Krivonogov, J. Mat. Sci., 27 (1992) pp. 4523-2530), etc.
The life of TBCs can be extended by blocking corrosive media and oxygen from entering the metal bond coat. In general, sealing the porous top coat (e.g. zirconia layer) by a laser is often selected. It is called laser glazing, laser sealing, or laser remelting in this field.. The surface after laser remelting is smoother and less porous than that of an as-sprayed top coat.
However, the laser remelting process has some disadvantages. When the surface is melted by a laser beam, the porous top layer is densified and becomes a liquid. The melted surface quickly solidifies as the laser beam passes. The solidification process starts from the surface and the liquid layer grows a great amount of columnar ceramic grains. This rapid cooling step results in appreciable thermal stresses, and it induces surface cracking and depressions in the top coat.